mysterious change in X authorization behavior

[Marty <martyb@ix.netcom.com>]

On one of my Sarge systems, if X access control is enabled, i.e.
"xhost" yields the following output:

   access control enabled, only authorized clients can connect

then if I su to root, X apps now start normally, as if I had previously
executed "xhost +".  All my sarge systems are up to date, but only one
exhibits this new behavior.  The only difference I see related to X
behavior, is that the changed system uses gdm instead of kdm.

The others still output the following error message after su'ing to root:

   Xlib: connection to ":0.0" refused by server

I'm not aware of any recent configuration changes that could account for
the change.  Since this is a change in security-related  behavior I am more
anxious than usual to get to the bottom of it.  Thanks for any clues about
what may have changed, and how.